[zer01]

Signing code can only do so much.

What Apple possesses is the somewhat unique ability to design a system that is actually secure by burning the key into the secure enclave and not allowing it to be updated. The only way someone would be able to get to it then is by attacking the physical hardware itself (which I'm sure an NSA-level attacker could do), but it would render this entire thing moot, as even Apple wouldn't be able to unlock the phone if it wanted.

I say unique because they can bake security into the actual hardware design, and tightly control how the entire thing works, which android & windows simply can't do. In order to trust your OS (and in turn, your signed software), you have to trust your hardware first. The security of the entire system falls apart if you can't trust your hardware.